The invention relates to a polymer optical fiber.
Polymer optical fibers are commercially available with circular cross-sections, resulting from the optical fibers being extruded in production, it being the circular cross-section which is the geometric shape easiest to produce with optimum precision.
When, however, at the user end the polymer optical fiber having a circular cross-section needs to be coupled to modules handling the functions of distributing and coupling the light into and out of the system the circular cross-section proves to be of advantage only conditionally since optimum lossless coupling to the module requires connecting parts having likewise a circular cross-section. Round structures can be machined by drilling and turning; however, producing circular cross-section structures by shaping techniques as employed preferably nowadays for cost-effective production of integrated optical components is difficult. Such components produced by a shaping technique contain optical waveguides which for production engineering reasons normally have a non-circular cross-section. When a polymer optical fiber having a circular cross-section is coupled to an integrated optical waveguide having the same cross-sectional area in a square shape, coupling losses of approx. 1 dB materialize when coupling is done from the optical fiber to the waveguide and then back again to the optical fiber, assuming a uniform illumination of the optical fiber. If the non-round waveguide cross-section is selected so large that it covers the round polymer optical fiber then although there are no losses in the transfer from the optical fiber to the waveguide, the losses in the reverse light path direction are all the more.
Also of a disadvantage with circular cross-sections is the reduced packing density. When circular cross-sections are employed densely packed, a large surface area portion remains unused, namely the surface area portion located between the circular cross-sections. Similar disadvantages materialize in the design of micro-optical splitters in which the input fiber of circular cross-section is usually mounted on the output fibers positioned side by side and having likewise a circular cross-section. The light incident in the gusset portion of the circular output fibers is lost and contributes towards the excess loss of the component.
It is the object of the invention to provide an optical fiber permitting a practically lossless exit of light guided in the optical fiber and having practically any desired cross-sectional shape.
For this purpose the polymer optical fiber in accordance with the invention is provided with a section having a circular cross-section as well as a section having a non-circular cross-section, a smooth transition from one cross-sectional shape to the other being provided between these two sections. The gist of the invention expressed simply is to reshape the polymer optical fiber at the desired location, for example in the region of its end face, such that the desired cross-sectional shape is attained. This cross-sectional shape may be configured polygonal, angular, square, elliptical, semicircular annular or otherwise as dictated merely by the existing requirements. When light is to be coupled into a waveguide of square cross-section from an optical fiber a square cross-section of the optical fiber is selected in the reshaped section, thus enabling the two cross-sections to be adjoined flush with no coupling loss.
With an optical fiber configured as such, greatly differing optical components can be formed. In accordance with one embodiment of the invention it is provided for that a splitter having one input fiber and two output fibers is formed, each fiber having a rectangular cross-section. In this way the resulting losses are reduced since the two output fibers can be arranged directly side by side without a portion existing in between which is not made use of for coupling.
In accordance with an alternative embodiment a splitter may also be formed having an input fiber and an output fiber, the input fiber having a circular cross-section and each of the two output fibers having a semicircular annular cross-section, the inner diameter of the two semicircular annular cross-sections equalling the outer diameter of the circular cross-section. The two semicircular annular optical fibers surround the input fiber in a contacting portion totally so that in this portion lossless coupling from the input fiber into the output fibers is attained.
In accordance with a further embodiment the component may be an array of several optical fibers, each of which has a rectangular cross-section. Due to the selected cross-sectional shape a very much higher packing density is achievable than with optical fibers having a circular cross-section.
The optical component may also be an illuminating element. In this arrangement the end face for light emission is freely designable to optical considerations. For example, by bundling several optical fibers a numerical display element is attainable in which the free end faces of the optical fibers have an elliptical cross-section.
A polymer optical fiber having a circular cross-section may be partially reshaped in accordance with the invention by a method involving the following steps: first the optical fiber is located by the section to be reshaped between two parts of a tool which in the closed condition define a recess whose cross-section corresponds to the cross-section of the reshaped optical fiber. The tool is then heated and the two parts of the tool are pressed together, the optical fiber thereby adapting to the recess between the two tool parts, after which the tool is cooled. Once the tool and the optical fiber contained therein have cooled the tool is opened and the optical fiber can be removed. By this method the optical fiber can be reshaped as desired at comparatively little cost and with high precision.
Provided outside of the tool is preferably a holding device for the optical fiber, this serving to prevent a shortening of the optical fiber when heated.
Advantageous aspects of the invention read from the subclaims.